Hvac system, a controller therefor and a method of measuring and managing ventilation airflow of an hvac system

ABSTRACT

A controller, an HVAC system employing the controller and a computer programmable product to implement a method of measuring and managing ventilation airflow of an HVAC system is disclosed. In one embodiment, the controller includes: ( 1 ) an interface configured to receive feedback data from the HVAC system, the feedback data corresponding to a pressure difference across the outdoor damper and an economizer damper position and ( 2 ) a ventilation director configured to determine a ventilation airflow rate of the HVAC system based on the pressure difference, the economizer damper position and economizer ventilation data of said HVAC system.

TECHNICAL FIELD

This application is directed, in general, to heating, ventilating and air conditioning (HVAC) systems, and more specifically, to determining and employing a ventilation airflow rate in HVAC systems.

BACKGROUND

(HVAC) systems can be used to regulate the environment within an enclosed space. Typically, an air blower is used to pull air (i.e., return air) from the enclosed space into the HVAC system through ducts and push the air (i.e., return air) back into the enclosed space through additional ducts after conditioning the air (e.g., heating, cooling or dehumidifying the air). Various types of HVAC systems may be used to provide conditioned air for enclosed spaces. For example, some HVAC units are located on the rooftop of a commercial building. These so-called rooftop units, or RTUs, typically include one or more blowers and heat exchangers to heat and/or cool the building, and baffles to control the flow of air within the RTU. Some RTUs also include an air-side economizer that allows selectively providing fresh outside air (i.e., ventilation or ventilating air) to the RTU or to recirculate exhaust air from the building back through the RTU to be cooled or heated again.

At least one type of an economizer includes two damper assemblies driven by a common actuator. The damper blades are linked such that when the outdoor damper is open, the return air damper is closed. When a building is occupied, the outdoor damper of the economizer is typically opened a small amount (e.g., ten to twenty five percent) to allow fresh air into the building to meet ventilation requirements. When the outdoor air is colder than the return air and cooling is needed, the outdoor damper is typically opened to a hundred percent to allow the cooler outdoor air to enter the building. These two functions of an economizer are often referred to as a ventilation mode and a free cooling mode, respectively.

Some HVAC systems use a powered ventilation damper that has a single damper assembly intended to bring only enough outdoor air to meet ventilation needs. Thus, for these economizers, the single damper assembly is an outdoor damper that is used to control the amount of fresh air that is allowed to enter a building through the HVAC system.

SUMMARY

In one aspect, a controller for an HVAC system is disclosed. In one embodiment, the controller includes: (1) an interface configured to receive feedback data from the HVAC system, the feedback data corresponding to a pressure difference across the outdoor damper and an economizer damper position and (2) a ventilation director configured to determine a ventilation airflow rate of the HVAC system based on the pressure difference, the economizer damper position and economizer ventilation data of said HVAC system.

In another aspect, a computer program product, including a non-transitory computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of measuring and managing ventilation airflow of an HVAC system having an economizer with an outdoor damper. In one embodiment the method includes: (1) receiving feedback data from the HVAC system, the feedback data corresponding to a pressure difference across the outdoor damper and an economizer damper position of the HVAC system, (2) applying the pressure difference and the economizer damper position to economizer ventilation data representing ventilation airflow rates of the HVAC system and (3) determining a ventilation airflow rate based on the applying, wherein the ventilation airflow rate corresponds to one of the ventilation airflow rates of the economizer ventilation data according to the pressure difference and the economizer damper position.

In yet another aspect, an HVAC system is disclosed. In one embodiment, the HVAC system includes: (1) an economizer having an outdoor damper and an actuator to move blades thereof, (2) a pressure sensor configured to determine a pressure difference across the outdoor damper and (3) a controller. The controller includes: (3A) an interface configured to receive feedback data from the HVAC system, the feedback data corresponding to the pressure difference and an economizer damper position; and (3B) a ventilation director configured to determine a ventilation airflow rate of the HVAC system based on the pressure difference, the economizer damper position and economizer ventilation data of said HVAC system.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment of an HVAC system constructed according to the principles of the disclosure;

FIG. 2 illustrates a block diagram of an embodiment of a controller constructed according to the principles of the disclosure;

FIG. 3 illustrates a block diagram of an embodiment of ventilation director constructed according to the principles of the disclosure;

FIG. 4 illustrates a flow diagram of an embodiment of a method of repositioning the dampers of an economizer according to the principles of the disclosure; and

FIG. 5 illustrates a flow diagram of an embodiment of a method of measuring and managing ventilation airflow of a HVAC system carried out according to the principles of the disclosure.

DETAILED DESCRIPTION

Knowing the ventilation airflow rate (i.e., airflow rate through the outdoor damper of the economizer) during the various operating modes of an economizer, such as the ventilation mode and the free cooling mode, is advantageous. When in the ventilation mode, the ventilation airflow rate provides verification that ventilation as required is being provided. If the ventilation airflow rate is too high, then energy may be wasted due to over ventilation. In a free cooling mode, knowing the ventilation airflow rate provides an indication of the energy savings provided by the economizer. Thus, determining the ventilation airflow of an HVAC system is often needed to verify that the system is providing the desired ventilation.

This disclosure provides a scheme for determining the ventilation airflow rate of an HVAC system employing feedback data of the operating HVAC system and the relationship of that feedback data to economizer ventilation data for the HVAC system. In one embodiment, a controller is disclosed that calculates the ventilation airflow rate employing the feedback data and the economizer ventilation data. The economizer ventilation data is developed from measured data obtained during manufacturing or engineering of the HVAC system. In one embodiment, the type of economizer ventilation data that is employed to calculate the ventilation airflow rate varies based on economizer damper position.

As disclosed in an embodiment herein, the feedback data employed to determine the ventilation airflow rate includes the economizer damper position and the pressure drop across the outdoor dampers of the economizer. In some embodiments, a supply airflow rate is also employed. Employing the supply airflow rate provide an enhancement that can increase the accuracy when the outdoor damper is 50% open or greater. Additionally, employing the supply airflow rate can increase the response time of control. Additionally, an outside air temperature and an elevation of the installed HVAC system can be employed. The economizer damper position can be determined from an actuator of the economizer. In one embodiment, position information from an actuator of the economizer is employed to determine and control the position of the damper blades of the economizer. Employing the position information from the actuator that moves the damper blades provides real-time data for accurately calculating outside airflow into the HVAC system.

In addition to determining the real-time ventilation airflow rate, a controller is disclosed that monitors and directs the economizer's dampers to achieve a user specified ventilation rate. The controller can also be configured to perform diagnostics and generate alarms to warn a user when the actual ventilation rate is above or below a desired value. In some embodiments, a controller and/or operating schemes are disclosed that compensate for hysteresis in the operation of an economizer actuator, automatically calibrate an actuator offset in the field, select economizer ventilation data based on the opening percentage of an economizer's outdoor dampers and compensate for temperature and elevation.

In one embodiment, a controller and operating schemes are also disclosed that employ the ventilation airflow rate that has been calculated to determine a prorated ventilation rate. The prorated ventilation rate can then be used to obtain a ventilation rate over a desired amount of time while reducing the run-time on the indoor fan or blower of the HVAC system. In one embodiment described herein, the HVAC controller monitors the fraction of time the compressor ran during the previous hour. Based on that runtime, the controller calculates a new higher ventilation rate which, when ventilating during only the compressor on time, provides the same amount of ventilation over an hour period as the original ventilation rate would provide with continuous operation. This enables the indoor fan to be turned off when the compressor is not running while still providing the required amount of ventilation. Turning the fan off when the compressor is not running will dramatically improve the ability to dehumidify. When the fan is running without the compressor on, water collected on the cooling coil evaporates, negating the dehumidification done when the compressor was running. Thus, disclosed herein are embodiments of dynamically adjusting a ventilation rate to allow fan off time.

FIG. 1 illustrates a block diagram of an embodiment of an HVAC system 100 constructed according to the principles of the disclosure. The system 100 includes an enclosure 101 (e.g., a cabinet) with openings for exhaust air, ventilation air, return air and supply air. The enclosure 101 includes exhaust vents 102 and ventilation vents 103 at the corresponding exhaust air and ventilation air openings. Within the enclosure 101, the system 100 includes an exhaust fan 105, economizer 110, a cooling element 120, an indoor fan or blower 130 and a heating element 140. Additionally, the system 100 includes a fan controller 150 and a HVAC controller 160. The fan controller 150 is coupled to the blower 130 via a cable 155. The cable 155 is a conventional cable used with HVAC systems. The HVAC controller 160 can be connected (not illustrated) to various components of the system 100, including a thermostat 119 for determining outside air temperature, via wireless or hardwired connections for communicating data. Conventional cabling or wireless communications systems may be employed. Also included within the enclosure 101 is a partition 104 that supports the blower 130 and provides a separate heating section.

The system 100 is an RTU. One skilled in the art will understand that the system 100 can include other partitions or components that are typically included within an HVAC system such as an RTU. While the embodiment of the system 100 is discussed in the context of a RTU, the scope of the disclosure includes other HVAC applications that are not roof-top mounted.

The blower 130 operates to force an air stream 170 into a structure, such as a building, being conditioned via an unreferenced supply duct. A return airstream 180 from the building enters the system 100 at an unreferenced return duct.

A first portion 181 of the air stream 180 re-circulates through the economizer 110 and joins the air stream 170 to provide supply air to the building. A second portion of the air stream 180 is air stream 182 that is removed from the system 100 via the exhaust fan 105.

The economizer 110 operates to vent a portion of the return air 180 and replace the vented portion with the air stream 175. Thus air quality characteristics such as CO₂ concentration and humidity may be maintained within defined limits within the building being conditioned. The economizer 110 includes an indoor damper 111, an outdoor damper 113 and an actuator 115 that drives (opens and closes) the indoor and outdoor dampers 111, 113 (i.e., the blades of the indoor and outdoor dampers 111, 113). Though the economizer 110 includes two damper assemblies, one skilled in the art will understand that the concepts of the disclosure also apply to those economizers or devices having just a single damper assembly, an outdoor damper assembly.

The controller 160 includes an interface 162 and a ventilation director 166. The ventilation director 166 may be implemented on a processor and/or a memory of the controller 160. The interface 162 receives feedback data from sensors and components of the system 100 and transmits control signals thereto. As such, the controller 160 may receive feedback data from, for example, the exhaust fan 105, the blower 130 and/or the fan controller 150, the economizer 110 and the thermostat 119, and transmit control signals thereto if applicable. One skilled in the art will understand that the location of the controller 160 can vary with respect to the HVAC system 100.

The interface 162 may be a conventional interface that employs a known protocol for communicating (i.e., transmitting and receiving) data. The interface 162 may be configured to receive both analog and digital data. The data may be received over wired, wireless or both types of communication mediums. In some embodiments, a communications bus may be employed to couple at least some of the various operating units to the interface 162. Though not illustrated, the interface 162 includes input terminals for receiving feedback data.

The feedback data received by the interface 162 includes data that corresponds to a pressure drop across the outdoor damper 113 and damper position of the economizer 110. In some embodiments, the feedback data also includes the supply airflow rate. Various sensors of the system 100 are used to provide this feedback data to the HVAC controller 160 via the interface 162. In some embodiments, a return pressure sensor 190 is positioned in the return air opening to provide a return static pressure. The return pressure sensor 190 measures the static pressure difference between the return duct and air outside of the HVAC system 100. In one embodiment, a supply pressure sensor 192 is also provided in the supply air opening to indicate a supply pressure to the HVAC controller 160. The supply pressure sensor 192 measures the static pressure difference between the return duct and the supply duct. Pressure sensor 193 is used to provide the pressure drop across outdoor damper 113 of the economizer 110. The pressure sensor 193 is a conventional pressure transducer that determines the static pressure difference across the outdoor damper 113. The pressure sensor 193 includes a first input 194 and a second input 195 for receiving the pressure on each side of the outdoor damper 113. The pressure sensors discussed herein can be conventional pressure sensors typically used in HVAC systems.

The HVAC controller 160 is configured to determine supply airflow according to conventional means. For example, in one embodiment, the HVAC controller 160 is configured to calculate the supply airflow rate based on a set of blower curves, fan power and fan speed.

Economizer damper position is provided to the HVAC controller 160 via the actuator 115. The actuator 115 is configured to rotate or move the indoor and outdoor dampers 111, 113, of the economizer 110 in response to a received signal, such as control signals from the HVAC controller 160 (i.e., the ventilation director 166). The actuator 115 is a conventional actuator, such as an electrical-mechanical device, that provides a signal that corresponds to the economizer damper position (i.e., blade angle of the outdoor damper 113 of the economizer 110). The signal is an electrical signal that is received by the ventilation director 166 which is configured to determine the relative angle of the outdoor damper 113 based on the signal from the actuator 115. A lookup table or chart may be used by the processor 117 to determine a relative blade angle with respect to an electrical signal received from the actuator 115. The angle can be based on (i.e., relative to) the ventilation opening of the HVAC system 100. In some embodiments, the economizer damper position can be determined via other means. For example, an accelerometer coupled to a blade (or multiple accelerometers to multiple blades) of the outdoor damper 113 may be used to determine the economizer damper position. The outdoor damper 113 is opened at 100 percent when the blades thereof are positioned to provide maximum airflow of ventilation air 175 into the system 100 through the ventilation opening. In FIG. 1, the blades of the outdoor damper 113 would be perpendicular to the ventilation opening or the frame surrounding the ventilation opening when opened at 100 percent. In the illustrated embodiment, the blades of the outdoor damper 113 would be parallel to the ventilation opening when opened at zero percent.

The ventilation director 166 is configured to determine an operating ventilation airflow rate of the HVAC system based on the static pressure difference across the outdoor dampers 113, the economizer damper position and economizer ventilation data. In some embodiments, the ventilation director 166 also employs the supply airflow rate to calculate the operating ventilation airflow rate. In one embodiment, using the supply airflow rate for the calculation is based on the economizer damper position being above 50 percent. In one embodiment, the economizer ventilation data is developed during manufacturing or engineering of the system 100 or similar type of HVAC systems. During development, a ventilation airflow rate is measured in, for example, a laboratory, at a variety of operating conditions. Various sensors and/or other type of measuring devices are employed during the development to obtain the measured data for the various operating conditions. Economizer ventilation data is developed from the measured data and loaded into the HVAC controller 160, such as a memory thereof. During operation in the field, the HVAC controller 160 (i.e., the ventilation director 166) receives the feedback data and calculates the ventilation airflow rate employing the feedback data and the economizer ventilation data. FIG. 3 provides a more detailed embodiment of a ventilation director 166.

The ventilation director 166 is further configured to adjust a position of the economizer 110 based on the economizer damper position and a desired ventilation airflow rate. The desired ventilation airflow rate can be preprogrammed into a memory of the HVAC controller 160 during manufacturing. In some embodiments, the desired ventilation airflow rate is entered into the HVAC controller 160 in the field during, for example, installation, a maintenance visit or a service visit. The ventilation director 166 generates a signal that directs the actuator 115 to adjust a position of the blades of the economizer 110 based on the desired ventilation airflow rate. In some embodiments, this signal represents a difference between the operating ventilation airflow rate and the desired ventilation airflow rate.

FIG. 2 illustrates a block diagram of an embodiment of a controller 200 constructed according to the principles of the disclosure. The controller 200 is configured to direct the operation of or at least part of the operation of an HVAC system, such as HVAC system 100. As such, the controller 200 is configured to generate control signals that are transmitted to the various components to direct the operation thereof. The controller 200 may generate the control signals in response to feedback data that is received from the various sensors and/or components of the HVAC system. The controller 200 includes an interface 210 that is configured to receive and transmit the feedback data and control signals. The interface 210 may be a conventional interface that is used to communicate (i.e., receive and transmit) data for a controller, such as a microcontroller.

The interface 210 may include a designated input terminal or input terminals that are configured to receive feedback data from a particular component. The controller 200 also includes a processor 220 and a memory 230. The memory 230 may be a conventional memory typically located within a controller, such as a microcontroller, that is constructed to store data and computer programs. The memory 230 may store operating instructions to direct the operation of the processor 220 when initiated thereby. The operating instructions may correspond to algorithms that provide the functionality of the operating schemes disclosed herein. For example, the operating instructions may correspond to the algorithm or algorithms that implement the method illustrated in FIG. 5. The processor 220 may be a conventional processor such as a microprocessor. The controller 200 also includes a display 240 for visually providing information to a user. The interface 210, processor 220 memory 230 and display 240 may be coupled together via conventional means to communicate information. The controller 200 may also include additional components typically included within a controller for a HVAC unit, such as a power supply or power port.

The controller 200 is configured to receive feedback data from the HVAC system including feedback data that corresponds to, for example, a pressure difference across an outdoor damper of an economizer, supply airflow rate and economizer damper position of the HVAC system. Additionally, the controller 200 is configured to determine an operating ventilation airflow rate of the HVAC system based on operating data, such as, the outdoor damper pressure difference, the supply airflow rate and the economizer damper position during operation. In some embodiments, the controller 200 also receives and employs condition data, such as, the outside ambient temperature and the elevation at the HVAC system, when calculating the ventilation airflow rate. The controller 200 calculates the ventilation airflow rate employing the feedback data, that includes the operating and condition data of the HVAC system, with the appropriate corresponding economizer data. In one embodiment, the economizer data is predetermined economizer ventilation data that is specific for particular HVAC systems or types of HVAC systems.

The controller 200 is further configured to adjust a position of an economizer of the HVAC system based on the economizer damper position and a desired ventilation airflow rate. In one embodiment, the controller 200 generates and transmits control signals to an actuator of the economizer to adjust the economizer damper position. In addition to the operation schemes disclosed herein, the controller 200 can be configured to provide control functionality beyond the scope of the present disclosure.

The controller 200 is also configured to generate alarms and status based on the ventilation airflow rate. In some embodiments, the controller 200 is configured to employ the ventilation airflow rate to determine a prorated ventilation airflow rate and direct the operation of an HVAC system based thereon.

FIG. 3 illustrates a block diagram of an embodiment of ventilation director 300 constructed according to the principles of the disclosure. The ventilation director 300 may be embodied as a series of operation instructions that direct the operation of a processor when initiated thereby. In one embodiment, the ventilation director 300 is implemented in at least a portion of a memory of an HVAC controller, such as a non-transistory computer readable medium of the HVAC controller. The ventilation director 300 includes a ventilation airflow determiner 310 and a ventilation changer 320.

The ventilation airflow determiner 310 is configured to calculate the operating ventilation airflow rate based on feedback data and economizer ventilation data. The economizer ventilation data is measured data that was obtained under various operating conditions in a laboratory environment. In one embodiment, the economizer ventilation data is specific for a particular type of HVAC system.

The ventilation airflow determiner 310 receives feedback data, such as operating data and condition data, from the HVAC system. The feedback data includes the outdoor damper pressure difference, the supply airflow rate and the economizer damper position. In one embodiment, the outdoor damper pressure difference is received from a pressure transducer, such as pressure sensor 193, that determines the pressure difference. In some embodiments the return duct pressure drop is employed for the outdoor damper pressure difference. The return duct pressure drop may be determined via conventional means and provided to the ventilation airflow determiner 310 for the outdoor damper pressure difference.

In typical applications, the return static pressure is within a range of a tenth of an inch to a half of an inch (0.1 inch to 0.5 inch) of water column. In some embodiments, the ventilation airflow rate ranges from 10 percent to 30 percent of the design airflow rate for the HVAC system. This 30 percent ventilation airflow rate of the designed system airflow rate can usually be obtained with a damper opening of 35 percent.

The elevation of the HVAC system can be stored in a memory of an HVAC controller. In one embodiment, the elevation is stored in the ventilation airflow determiner 310. The elevation is a parameter that is typically entered by a user during initial setup. The elevation may be entered, for example, during installation or a service visit. The outdoor temperature can be provided by a thermometer associated with the HVAC system. As discussed with respect to FIG. 1, the supply airflow rate can be provided by conventional means the economizer damper position can be provided from feedback data of an economizer actuator.

The ventilation airflow determiner 310 is configured to calculate the ventilation airflow rate employing a combination of equations, feedback data and the economizer ventilation data. In some embodiments, the economizer ventilation data is stored in look-up tables.

The ventilation airflow determiner 310 calculates the ventilation airflow rate differently according to the current economizer damper position. When the current economizer damper position is 50 percent or less, the ventilation airflow determiner 310 employs Equation 1 to calculate the ventilation airflow rate.

Ventilation Airflow Rate=1096*CA(ΔP/ρ)^(1/2)  (Equation 1)

In Equation 1, ΔP is the outdoor damper pressure difference and CA is the damper effective open area expressed in squared feet (i.e., ft²). The value 1096 is a conversion constant that is used to make the measurement units more useable. The effective open area CA is calculated employing a flow coefficient table of the economizer ventilation data established for the HVAC system. Flow coefficient data is a parameter developed from testing of HVAC systems that is a function of damper position and relates outdoor damper position to the effective open area CA. The ventilation airflow determiner 310 is configured to select the appropriate flow coefficient data from the economizer ventilation data based on the economizer damper position. For a current economizer damper position that is 50 percent or less, a first table of flow coefficient data is selected and employed. Table 1 is an example of a flow coefficient table that is selected for an economizer damper position less than or equal to 50 percent. The values in Table 1 are unique for a particular economizer damper assembly and are provided as an example. The flow coefficients for two HVAC models, Model A and Model B, are provided in Table 1. One skilled in the art will understand that flow coefficient tables for other particular HVAC systems can be developed and stored with a controller of the particular HVAC systems. In some embodiments, the ventilation airflow determiner 310 is configured to determine the effective air opening CA by interpolation of the data in a flow coefficient table such as Table 1.

TABLE 1 Flow Coefficients for Economizer Damper Position Equal To or Less Than Fifty Percent CA CA % OPEN MODEL A MODEL B 0 0.0 0.0 5 0.055736 0.04812 10 0.083934 0.095381 15 0.113264 0.125026 20 0.151411 0.166996 25 0.208313 0.219794 30 0.278474 0.289318 35 0.354823 0.390838 40 0.460648 0.538106 45 0.588303 0.718347 50 0.722145 0.942691

In Table 1, % Open represents the outdoor damper blade position relative to the frame of the HVAC system at the ventilation opening. In one embodiment, the % Open is calculated using an actuator feedback signal. The relationship between the % Open and the actuator feedback signal is typically dependent on the characteristics of the actuator and the design of the economizer. In one embodiment, the relationship between % Open and the actuator feedback signal is represented with Equation 2.

% Open=100×(V _(feedback) −V _(offset))/8  (Equation 2)

V_(feedback) and V_(offset) correspond to the type of actuator that is used. V_(feedback) is the feedback voltage output by the actuator. V_(offset) is a voltage value that corresponds to a fully closed economizer. In one embodiment, V_(offset) is nominally two volts, V_(feedback) is two volts when the damper is 0% open and V_(feedback) is ten volts when 100% open. The number 8 in Equation 2 is a conversion constant that is specific to the type of actuator employed.

V_(offset) may vary from part to part. For example, in one embodiment V_(offset) can vary between 2.1 volts to 2.75 volts with a closed damper. As such, instead of using a fixed offset based on the actuator specification, in some embodiments a measured offset is used. To determine the measured offset, the actuator is commanded to go to its minimum position during calibration. After waiting the amount of time required to move to its minimum position, the ventilation airflow determiner 310 measures the feedback voltage. If the feedback voltage is within the normal variation of offset voltage, the current feedback is recorded as the offset voltage. If the feedback voltage is not within the normal variation of offset voltage, an error code is generated and the default offset is used.

During operation, hysteresis in the relationship between the actuator feedback signal and the actual position of the economizer damper blades can occur. As such, the ventilation director 300 (i.e., the ventilation airflow determiner 310 or the ventilation changer 320) can reposition the damper blades. The flow diagram of FIG. 4 illustrates an embodiment of such a method.

Returning to Equation 1, p is the density of air entering the outdoor damper. In one embodiment, the ventilation airflow determiner 310 calculates the air density p employing Equation 3.

P=0.075((460+64)/(460+T _(OD)))(P _(atm)/14.696)  (Equation 3)

In Equation 3, T_(OD) is the outdoor temperature in Fahrenheit and P_(atm) is the atmospheric pressure calculated by Equation 4.

P _(atm)=14.696*(1-6.876E−6*ALT)^(5.25588)  (Equation 4)

In Equation 3, ideal gas relationships are being used to correct air density for temperature and pressure variations. 0.075 is a reference density of air at 64F and 14.696 psia (sea level). The first term 460+64/46+T corrects the reference density for temperature (460 is used to convert the temperature to the absolute ranking scale). The term P_(amt)/14.696 corrects for atmospheric pressure. Thus, the density is calculated using T_(OD) and P_(atm) and ideal gas relationships. Equation 4 is a standard equation used by the national weather service to calculate atmospheric pressure as a function of elevation wherein the terms have been converted for US units.

In Equation 4 ALT is the elevation of the HVAC system in feet and is a user entered parameter. An elevation of 650 feet, which is approximately the median elevation, is entered as a default elevation. This can be entered during manufacturing of an HVAC system or when programming a controller of the HVAC system. Additionally, a default outdoor temperature of 70 degrees Fahrenheit may also be used. Calculating the air density based on elevation and temperature increase the accuracy of the ventilation measurement across wide temperatures and at high altitudes.

When the current economizer damper position is greater than 50 percent, the ventilation airflow determiner 310 employs a different flow coefficient table to calculate the ventilation airflow rate. For example, Table 2 represents a flow coefficient table for a particular type of HVAC system when the current economizer damper position is greater than 50 percent. In some embodiments, the ventilation airflow determiner 310 is configured to determine the percentage of outdoor air by interpolation of the data in a flow coefficient table such as Table 2. Once the percentage of outdoor air is known, the ventilation airflow determiner 310 multiplies the percentage of outdoor air by the total supply airflow to determine the ventilation airflow rate. As with Table 1, the flow coefficients for two different models of HVAC systems are provided as an example.

TABLE 2 Flow Coefficients for Economizer Damper Positions Greater Than Fifty Percent % OD AIR % OD AIR % OPEN MODEL A MODEL B 50 65.3 65.3 60 79 79 70 88.2 88.2 80 95.1 95.1 90 97 97 100 97 97

Thus, the ventilation airflow determiner 310 selects the appropriate flow coefficient table to employ based on the current economizer damper position and determines the operating ventilation airflow rate that is provided to the ventilation changer 320. The ventilation changer 320 receives the operating ventilation airflow rate and a desired ventilation airflow rate. Based on these received airflow rates, the ventilation changer 320 adjusts the economizer damper position to obtain the desired ventilation airflow rate. The desired ventilation airflow rate may be received via a user interface, such as a touch screen or keypad, associated with an HVAC controller or the ventilation director 300. In one embodiment, the desired ventilation airflow rate is stored and received from a memory, such as the memory of an HVAC controller. The various ventilation airflow rates may be provided to a user via a display of an HVAC controller.

The ventilation changer 320, therefore, uses the ventilation airflow rate determined above to automatically adjust the damper actuator position command delivered to the actuator to achieve a user specified ventilation rate. In some embodiments, the ventilation changer 320 is configured to minimize movement of the actuator. As such, concerns about reliability limitations of an economizer actuator are minimized. Accordingly, in some embodiments, a ventilation changer 320 is configured to change the damper position once per a designated time. In some embodiments, the ventilation changer 320 is configured to change the damper position only once in every 10 minutes. In other embodiments, the ventilation changer 320 is configured to change the damper position when the operating state of the fan system has changed. The basis for determining when to change the damper position and the designated time for changing the damper position are adjustable.

In some embodiments, designated events may be predetermined to use as a basis for determining when to change the damper position. For example, a change in supply air fan speed and a change in ventilation setpoint can be used to trigger a change in damper position. In one embodiment, the ventilation changer 320 is configured to continuously integrate the error between the actual ventilation rate and the desired rate when waiting to make a control move. In one embodiment, the ventilation changer 320, when determining it is time to make a control move, determines the next position of the damper blades of the outdoor damper with following procedure:

1) Calculate an integral offset of the actuator where the integral offset=−1*Integrated Error/Integral Gain. If the absolute value of the integral offset is greater than the desired ventilation rate, then the integral offset is set equal to the integral offset multiplied by the desired ventilation rate divided by the absolute value of the integral offset. To prevent over opening or over closing the damper, a ventilation rate more than twice the normal ventilation rate may not be employed.

2) Calculate the new ventilation target airflow using the following by adding the desired ventilation rate and the integral offset together.

3) Calculate the current ventilation airflow rate using a procedure defined above with respect to the ventilation airflow determiner 310.

4) Acquire the current outdoor damper pressure difference.

5) Acquire the current supply airflow.

6) Acquire the current economizer damper position.

7) Calculate the new predicted damper pressure difference employing the following equation, Equation 5, wherein CurrentDP is the current economizer damper position, CurrentCFM is the current supply airflow and VentTarget is the ventilation target. For Equation 5, the ventilation changer 320 can employ the return duct static pressure difference as the pressure difference across the outdoor damper. Typically, the return duct pressure drop is proportional to the square of the airflow rate through the return duct. In this embodiment, the ventilation changer 320 assumes that the airflow through the return duct is equal to the supply airflow rate minus the ventilation airflow rate.

newDP=CurrentDP*((CurrentCFM−VentTarget)/(CurrentCFM−CurrentVent))̂2  (Equation 5)

8) Calculate the new CA employing Equation 6.

newCA=VentTarget/(newDP)̂0.5  (Equation 6)

9) Use the economizer ventilation data (such as Table 1) to determine the economizer damper position, i.e., the new damper position associated with the new CA, and determine the position difference between the new damper position and the current damper position. If the absolute value of the position difference is less than Deadband (i.e., less than the steps at which the actuator can move, such as 1.5% step), then set the new damper position as the new damper position. Otherwise, set the new damper position equal to the current position.

The ventilation director 300 (i.e, either the ventilation airflow determiner 310 or the ventilation changer 320 or a combination thereof) can also perform diagnostics, detect faults with the economizer and generate alarms. The alarms could be visually presented on a display of a controller and/or communicated to a monitor or monitoring service. An audible alarm may also be generated. The diagnostics can be used to warn a user of a fault which could cause an inaccurate measurement of ventilation airflow. An example of an alarm resulting from receiving feedback data from the economizer actuator includes Damper Stuck. Damper Stuck can be determined by comparing actuator feedback position to command position. During operation of the damper actuator, the feedback position of the damper is compared with the desired position. Once the actuator has stopped moving, if the feedback position in not within a prescribed tolerance of the desire position, the algorithm indicates a fault. The ventilation director 300, will continue to monitor the feedback position and automatically clear the fault should the feedback start to match the command.

In one embodiment, the ventilation director 300 is also configured to perform damper pressure sensor diagnostics. Based on normal operating data that can be stored in an HVAC controller, the ventilation director 300 can compare the outdoor damper pressure difference with the percent of damper opening and generate an alarm if the measured pressure is out of range compared to the stored operating data. An error can be recorded and an alarm generated based on the comparison.

The ventilation director 300 can also be configured to employ the ventilation airflow rate to determine the damper position necessary to deliver required ventilation only when the compressor is running. As such, humidity problems associated with a continuous fan can be reduced or eliminated and operation of the HVAC system can still comply with Indoor Air Quality standards established by governing bodies, such as the ASHRAE 62.1 standard. In one embodiment, the ventilation director 300 is configured to determine a prorated ventilation airflow rate and deliver the required ventilation as described below. An hour is used in the embodiment discussed below but other amounts of time may also be used in different embodiments.

1) At the beginning of each hour:

-   -   a. determine the fraction of compressor on time during the past         hour (i.e., runfrac);     -   b. calculate the required ventilation rate (when compressor is         on using Equation 7 employing runfrac and the ventilation rate         when the compressor is on continually (Qvent_(CONT)). The         constant 1.2 in Equation 7 is a margin of safety which ensures         the correct amount of ventilation is delivered even if the         compressor runs 20% less than the previous hour.

Qvent_(compOn)=1.2(Qvent_(CONT)/runfrac)  (Equation 7)

2) When the compressor is on, set the ventilation controller setpoint to Qvent comp ON.

3) When the compressor is off, set the ventilation setpoint to 0.

4) Integrate the amount of ventilation airflow delivered over an hour. If the integrated amount exceeds Qvent cont*60 then set the ventilation setpoint=0.

Turning now to FIG. 4, illustrated is a flow diagram of an embodiment of a method 400 of repositioning the dampers of an economizer according to the principles of the disclosure. In some embodiments, hysteresis results in the relationship between the actuator feedback signal and the actual position of the economizer damper blades. In some embodiments, the hysteresis can be significant enough to cause a ten percent error in the relationship between the actuator feedback and the damper blade position. The method 400 can be employed to correct this problem. In one embodiment, a ventilation airflow determiner is configured to perform the method 400. The method 400 represents an algorithm that can be implemented as a series of operating instructions.

The method 400 begins in a step 405 with a change in the position of the dampers being desired. In a decisional step 410, a determination is made if the new desired damper position is less than the current damper position. Thus, step 410 includes comparing the current damper position (e.g., the current percentage of opening) to the desired damper position (e.g., the desired percentage of opening). If the desired position is less than the current position, then the method continues to step 420 where the actuator is closed directly to the desired position. If the desired position is not less than the current position, then the method continues to step 430 where the actuator is opened to the desired position plus an actuator specific buffer. In one embodiment, the actuator specific buffer is based on the amount of slack of the drive train of the actuator. In some embodiments, the actuator specific buffer is 1.5 volts. The method 400 then ends in a step 440 where the actuator is closed to the new desired position.

One skilled in the art will understand that the buffer employed can vary based on the type of actuator and the actual installation. The value (e.g., voltage) of the buffer can be determined during calibration. The method 400 represents compensating for hysteresis employing a final close operation (step 440). A similar compensation can be performed by ending in an open operation. For example, in step 430, the actuator could be opened to the new position with the addition of a negative buffer (e.g., −1.5 volts). As such, in step 440, the actuator would be opened to the new position.

FIG. 5 illustrates a flow diagram of an embodiment of a method 500 of measuring and managing ventilation airflow of a HVAC system carried out according to the principles of the disclosure. The method 500 may be carried out under the direction of a computer program product. In one embodiment, a controller of an HVAC system is employed to carry out the method 500. The method 500 begins in a step 505.

In a step 510, feedback data is received from an HVAC system. In one embodiment, the feedback data corresponds to the pressure difference across an outdoor economizer damper and economizer damper position of the HVAC system. Additionally, the feedback data may include the supply airflow rate. The feedback data is real time data obtained during operation of the HVAC system.

The feedback data is applied to economizer ventilation data in a step 520. The feedback data applied may include the outdoor economizer damper pressure difference, the supply airflow rate and the economizer damper position. The economizer ventilation data represents ventilation airflow rates of the HVAC system and is based on measured data obtained before installation of the HVAC system.

In a step 530, an operating ventilation airflow rate is calculated based on the feedback data and the corresponding economizer ventilation data.

A desired ventilation airflow rate is received in a step 540. In a step 550, a position of the economizer is adjusted based on the economizer damper position and the desired ventilation airflow rate. In some embodiments, the adjustment is zero when the operating ventilation airflow rate is at or within a designated percentage of the desired ventilation airflow rate. In some embodiments, the desired airflow rate is entered by a user in the field. In other embodiments, the desired airflow rate is predetermined and established before or during installation. In these embodiments, the desired airflow rate can be changed after installation. The method 500 ends in a step 560.

The above-described methods may be embodied in or performed by various conventional digital data processors, microprocessors or computing devices, wherein these devices are programmed or store executable programs of sequences of software instructions to perform one or more of the steps of the methods, e.g., steps of the method of FIG. 5. The software instructions of such programs may be encoded in machine-executable form on conventional digital data storage media that is non-transitory, e.g., magnetic or optical disks, random-access memory (RAM), magnetic hard disks, flash memories, and/or read-only memory (ROM), to enable various types of digital data processors or computing devices to perform one, multiple or all of the steps of one or more of the above-described methods, e.g., one or more of the steps of the method of FIG. 5. Additionally, an apparatus, such as dedicated HVAC controller, may be designed to include the necessary circuitry to perform each step of the methods disclosed herein.

Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments. 

What is claimed is:
 1. A controller for a heating, ventilating and cooling (HVAC) system having an economizer with an outdoor damper, comprising: an interface configured to receive feedback data from said HVAC system, said feedback data corresponding to a pressure difference across said outdoor damper and an economizer damper position; and a ventilation director configured to determine a ventilation airflow rate of said HVAC system based on said pressure difference said economizer damper position and economizer ventilation data of said HVAC system.
 2. The controller as recited in claim 1 wherein said ventilation director is further configured to adjust a position of said economizer based on said economizer damper position and a desired ventilation airflow rate.
 3. The controller as recited as recited in claim 1 wherein said ventilation director is configured to determine said ventilation airflow rate based on said economizer ventilation data, said pressure difference, said economizer damper position and a supply airflow rate of said HVAC system.
 4. The controller as recited in claim 3 wherein said economizer ventilation data is developed based on measured data obtained while operating said HVAC system over multiple operating conditions during manufacturing or engineering thereof.
 5. The controller as recited in claim 1 wherein said economizer has a single damper assembly.
 6. The controller as recited in claim 1 wherein said economizer damper position indicates a blade angle of said outdoor damper and is based on a feedback signal from an actuator of said economizer.
 7. The controller as recited in claim 6 wherein said ventilation director is further configured to compensate for hysteresis between said feedback signal and an actual position of blades of said outdoor damper.
 8. The controller as recited in claim 1 wherein said ventilation director is further configured to employ an outside air temperature and an elevation associated with said HVAC system to determine said ventilation airflow rate.
 9. The controller as recited in claim 1 wherein said ventilation director is configured to select data from said economizer ventilation data to calculate said ventilation airflow rate based on said economizer damper position.
 10. The controller as recited in claim 1 wherein said ventilation director is further configured to determine a prorated ventilation rate based on said ventilation airflow rate.
 11. The controller as recited in claim 1 wherein said ventilation director is further configured to generate an alarm or a status of said economizer based on said feedback.
 12. A computer program product, comprising a non-transitory computer usable medium having a computer readable program code embodied therein, said computer readable program code adapted to be executed to implement a method of measuring and managing ventilation airflow of a heating, ventilating and air conditioning (HVAC) system having an economizer with an outdoor damper, said method comprising: receiving feedback data from said HVAC system, said feedback data corresponding to a pressure difference across said outdoor damper and an economizer damper position of said HVAC system; applying said pressure difference and said economizer damper position to economizer ventilation data representing ventilation airflow rates of said HVAC system; and determining a ventilation airflow rate based on said applying, wherein said ventilation airflow rate corresponds to one of said ventilation airflow rates of said economizer ventilation data according to said pressure difference and said economizer damper position.
 13. The computer program product as recited in claim 12 wherein said method further comprises receiving a desired ventilation airflow rate and adjusting a position of said economizer based on said economizer damper position and said desired ventilation airflow rate.
 14. The computer program product as recited in claim 12 wherein said economizer ventilation data is developed based on measured data obtained while operating said HVAC system over multiple operating conditions during manufacturing or engineering thereof.
 15. The computer program product as recited in claim 12 wherein said economizer damper position indicates a blade angle of said outdoor damper and is based on a feedback signal from an actuator of said economizer.
 16. The computer program product as recited in claim 15 wherein said method further comprises compensating for hysteresis between said feedback signal and an actual position of blades of said outdoor damper.
 17. The computer program product as recited in claim 12 wherein said determining further includes employing a supply airflow rate of said HVAC system.
 18. The computer program product as recited in claim 12 wherein said method further includes determining a prorated ventilation rate based on said ventilation airflow rate.
 19. A heating, ventilating and cooling (HVAC) system, comprising: an economizer having an outdoor damper and an actuator to move blades thereof; a pressure sensor configured to determine a pressure difference across said outdoor damper; and a controller, comprising: an interface configured to receive feedback data from said HVAC system, said feedback data corresponding to said pressure difference and an economizer damper position; and a ventilation director configured to determine a ventilation airflow rate of said HVAC system based on said pressure difference, said economizer damper position and economizer ventilation data of said HVAC system.
 20. The HVAC system as recited in claim 19 wherein said ventilation director is further configured to adjust a position of said economizer based on said economizer damper position and a desired ventilation airflow rate. 